Sleep apnea, the cessation of breathing due to unknown causes, continues to tragically affect the lives of families with infants. Clinical data suggests that the incidence of infant mortality attributable to breathing cessation from unknown causes, also know as sudden infant death syndrome or crib death, is approximately one per 500 apparently normal infants.
For some time it has been recognized that an infant experiencing sleep apnea can be saved if the condition is immediately detected, and steps toward resuscitation are promptly taken. The prior art is replete with monitoring/alarm devices intended to accomplish this goal by detecting the cessation of breathing and subsequently sounding an alarm.
Of the various types of monitoring devices disclosed in the prior art, the only type which has achieved any appreciable commercial success utilizes electrodes which are physically attached to the infant's chest. These attached electrode type devices are capable of monitoring even the most subdued breathing motions of an infant. Unfortunately, the high cost of these monitors precludes their use in the average home. This same high cost also practically limits the use of these monitors in hospitals to those infants considered to be at high risk. Even when home monitoring is prescribed for a high risk infant, the substantial rental fees that are charged for these attached electrode type monitoring devices often discourage monitoring for the full 18 month period during which infants are generally considered to be susceptible to crib death.
In addition, all attachable monitoring devices, including the attached electrode type devices described above, have inherent shortcomings. Each time the infant is put down to (or picked up from) sleep, the sensing means must be connected to (or disconnected from) the infant. For example, in the previously described attached electrode types, the electrodes must be taped or otherwise secured to the infant's body. In addition to the inconvenience of attaching and detaching these devices, attached monitoring devices often cause discomfort for the infant. Moreover, attached devices are readily tampered with by older children or by the infant himself; or the devices may become unattached during normal sleeping movement of an infant. Detachment of such attachment monitoring devices, of course, renders them inoperative.
In order to overcome the inherent shortcomings of the attached type monitoring devices described above, numerous prior art attempts have been made to develop nonattached devices for detecting the cessation of infant breathing. These nonattached devices typically include an electromechanical transducer element which modifies an electrical signal in response to detected mechanical movement. For example, in U.S. Pat. No. 3,991,746 to Hanna and U.S. Pat. No. 3,926,177 to Hardway et al, first and second flexible conductive sheets separated by an electrically insulating sheet are placed beneath a subject. When a carrier frequency is applied to the conductive sheets, movement of the subject varies the capacitance between the conductive sheets. This variation in capacitance is detected to provide an indication of the subject's movement. Other monitoring systems, such as disclosed in U.S. Pat. No. 3,727,606 to Sielaff have used fluid filled mattresses and have detected variations in fluid pressure produced by movement of the subject to provide an indication of breathing.
Virtually all of the prior art monitoring devices are capable of detecting gross movement. However, in order to be effective in detecting apnea in infants, a device must be able to reliably detect and monitor relatively minor movements which result from infant breathing. This task is compounded when the transducer element area is expanded to cover any reasonable size sleeping area, even, for example, an expansion adequate to cover an infant's crib. Typically, increasing the size of the detection area lowers the overall transducer element sensitivity. Moreover, sheets, mattress covers and clothing worn by an infant all tend to mask the infant's breathing movement, making detection of such movement more difficult.
Unfortunately, simply increasing the gain of the signal conditioning electronics used in connection with prior art nonattached monitoring transducers does not provide a satisfactory solution. Electrical background noise is amplified by the same amount as movement induced signals, and such amplified background noise may be falsely interpreted by the electronics as movement of an infant. Failure to distinguish electrical background noise from movement may falsely indicate that the infant is moving when no such movement has occurred, and may preclude activation of an alarm. This, of course, can be fatal to an infant experiencing apnea. On the other hand, reducing the gain to avoid detection of electrical background noise reduces the ability of the system to detect subdued breathing movements of an infant and results in false alarms. Such false alarms, often occurring in the middle of the night, further disrupt the lives of already worried parents.
One prior art device which attempts to overcome the problem of transducer element insensitivity resulting from enlarging the detection area to cover a reasonable size sleeping area is disclosed in U.S. Pat. No. 3,836,900 to Mansfield. Mansfield discloses a device which utilizes a number of layers of resilient resistor material arranged so that movement of one layer relative to a contiguous layer changes the contact resistance between layers. The sensor material in Mansfield is arranged in a parallel fashion to cover a normal size sleeping area. While that manner of paralleling the sensor material dampens the response of the transducer element, it has a much more significant and positive effect of increasing the signal amplitude being monitored. However, this method of enhancing the signal presents new problems associated with the edge effects of the sensor material. In other words, the relative movement between the layers of sensor material generates variable and unpredictable contact resistance. This causes undesirable spurious signals, which may be misinterpreted by the electronics as movement of a subject being monitored. As a consequence, the signal conditioning circuitry of Mansfield must include a means for reducing the amplification of the transducer element signal in order to suppress or attenuate those edge effects which result from the resilient material slowly returning to its predisturbed state. Such an attenuation control would have to be properly adjusted by the user according to the size and weight of the infant being monitored, because the magnitude of the edge effects is directly dependent upon these two factors. Too much attenuation results in false signals, and too little attenuation may prevent an apnea condition from being detected.